Unfortunately at this point the only option you have is to find a new aftermarket or OEM catalytic converter for your 1998 Nissan Frontier. You should be able to find one online for around $300.00.
It is not necessary that you purchase a new CAT from the dealer. There are aftermarket catalytic converter companies such as CATCO that make inexpensive and California compliant (OBDII) CATs for your Nissan Frontier.
You can order one and have a muffler shop install it for you, or you can find a muffler shop that has an OEM CAT for your truck in stock.